Allow setting tabindex

[lmanzurv]

I'm using combobox and listbox and I'd like to pass the tabindex so I can control how the form is navigated. However, both the combobox and listbox are ignoring the tabindex property and set it to 0 always, as seen here and here.

Would it be possible to have it so it only sets the tabindex to 0 if no tabindex has been set in the input/button? or perhaps pass it as an init option.